Various types of circuitry are currently employed to interface with local area computer networks. Depending upon the type of local area network employed peripheral devices are selected that appropriately interface with the network employed. Currently, two major types of network are token ring and Ethernet.
These peripheral devices employ very complex circuitry. When this circuitry is turned "on" (or powered up), it is conventional to perform some initialization and self-testing. During this initialization and/or self-testing the circuitry employs an internal clock to provide any necessary clock signals. However, at one or more points in time, after start-up, the circuitry must synchronize itself with an external clock. Depending upon the type of network, synchronization with the external clock may also be required as a part of normal operations.
A computer network generally employs a clock to synchronize communications between the various peripheral devices attached to it. Currently, a token ring network provides a clock signal to the network as long as the network is operating. An Ethernet network, on the other hand, only provides a clock signal when data is being transmitted over the network. Thus, the peripheral devices use a control signal, generally called "carrier sense", to determine whether there is information available from the network, such as its clock signal. For an Ethernet network, the carrier sense signal and network clock signal may start with or slightly precede the start of a packet of data and may terminate with or slightly after the end of the data packet.
Thus, there is a need for clock control circuitry that enables the circuitry of the peripheral device to dynamically switch between an internal clock and an external clock.
These and other disadvantages of the prior art are overcome by the present invention, however, and improved dynamic clock switching circuitry and method are provided.